1. Field of the Invention
The invention relates in general to transceivers having generic data and control interface between a RF-unit (RF=radio frequency) and a base-band unit (BB=base band) of a fast frequency-hopping transceiver especially in an OFDM based communication system (OFDM=orthogonal frequency division multiplexing). The transceiver may be a UWB based transceiver (UWB=ultra wideband).
This invention is specifically related to realizing a multi-band OFDM UWB-transceiver that obeys the evolving multi-band OFDM standard (MB-OFDM). This multi-band OFDM standard is based on fast frequency hopping between frequency bands.
2. Technical Background
The multi-band OFDM communication is based on multi-point to multi-point type of communication. In this type of communication a set of transceivers are coupled together defining a local network, e.g. a wireless local area network (WLAN). Several transceivers belonging to the same wireless local area network use the same frequency spectrum and thus use the same data transmission channel by means of time domain sharing. This method is referred to as TDMA (TDMA=time domain multiple access).
In this communication system at any specific time only one transceiver is allowed to transmit data. Accordingly, the data communication between the different transceivers is burst like, that is the transmitting transceiver sends the data information to the receiving transceiver by means of several data transmission bursts. For supporting the receiving transceiver to identify these data transmission bursts and for extracting the delivered data information therein the transmitting transceiver sends a predefined preamble preceeding the data portion of the data transmission burst. Every receiving transceiver within the wireless local area network has no previous knowledge of the received bursts. This lack of knowledge includes the timing of the burst, the identity of the sender, and consequently the received signal level.
However, the receiving transceiver comprises a preamble detection unit that identifies the preamble and thus identifies the data transmission bursts. The transceiver uses further the preamble for estimating data transmission and channel parameters such as channel response and carrier and timing offsets that are needed for the extraction of the data information extraction. The predefined preamble preceding every data transmission bursts assists the receiver to detect the existence of this preamble and to extract parameters that enable demodulation, decoding and data extraction. Among the extracted parameters within the preamble there are parameters that are significant for the tuning of the analog front end (AFE) of the transceiver such as the level of the AGC unit (AGC=automatic gain control) and the timing of a so-called frequency hopping.
To perform this task an interface arranged between the AFE-unit of the transceiver and the base-band unit of the transceiver is needed which is designed to enable fast detection and acquisition of the necessary parameters within the preamble detection within a relatively short time period.
A radio communication link consists at least of the following communication (interface) units: a transmitter performing the pre-equalization and a receiver performing an automatic gain control (AGC), an analog signal level detection, a DC-offset cancellation. Moreover, both units (i.e. the transmitter and the receiver) have to perform frequency hopping which is in particular difficult for the receiving unit. To perform the above mentioned task, consequently it is a challenge to fulfill the following requirements of the transmitter and receiver, respectively.
Pre-Equalization (Within Transmitter):
In terms of the transceiver performance—that is the achievable data rate and/or the achievable range—it is desired to transmit as much power as allowed. However, the regulation authorities such as the FCC in the USA limit the allowed transmitted power spectrum density (PSD). Therefore, it is desired to transmit the maximum power in each band without violating the regulation limit.
The analog transmitting path including the antenna is characterized by a different gain in each transmitted frequency band. Therefore, for obtaining improved performances of the data transmission it is necessary and advantageous to provide a transmitter having a different gain for every transmitted frequency band. For the realization of this transmitter a fast data communication between the two units of the communication system is needed.
AGC (Within Receiver):
A similar, however more complex problem exists with the receiving path and especially with the AGC-unit therein. Here, the problem is more severe since the received signal level is not known in advance, but should be tuned within the preamble of the received data transmission burst. The base-band unit of the transceiver should carry out several tasks within processing of the preamble including preamble detection, timing acquisition, carrier offset acquisition, etc. This should be done on the one hand within a time limited preamble and on the other hand under very challenging noise conditions. The conditions are severe since the UWB typically relies on processing gain of some spreading modulation resulting in data rates that are significant lower than the utilized bandwidth.
Analog-Signal-Level Detection (within Receiver):
An additional challenge for the AGC mechanism is the fact that signal measurement at the digital domain—that is after the analog to digital conversion (ADC) of the transmitted signal—does not necessarily determine the optimal setup of the programmable gain amplifiers (PGA) along the receive path within the AFE-unit. The reason for this ambiguity results from the mutual detection of the in-band signal and an attenuated out of band signal without the ability to differentiate between these two signals. Miss-selection of the PGA-levels may result in signal compression along the receiving path.
DC Offset-Cancellation (Within Receiver):
Sometimes the AFE-receiver causes a DC-offset at the ADC input to a greater or lesser extent. However, such a DC offset either reduces the effective dynamic range of the ADC or potentially saturates the signal in a way that even no signal is received. To prevent this effect there is a need for a DC offset cancellation in the receiving path. However, treating a DC offset for the multi-band OFDM is a challenging task as a result of the very fast band hopping (the DC offset is typically band dependent) and since the offset depends on the setup of the programmable gain amplifiers (PGA) along the receiving path and should therefore be acquired on a burst basis within the preamble signal.
Hopping Commands:
The base-band unit uses the preamble signal to identify the hopping time of the received signal, and additionally selects the hopping timing for enabling good performance. Therefore the base-band unit should command the AFE-unit by means of a hopping signal. Here, a main challenge lies in the provision of an accurate hopping time with a low rate signal.
As mentioned above, UWB-communication systems offer in contrast to previous digital communication systems significant higher data rates, however at the price of larger bandwidths, shorter settling times and higher frequency hopping rates. For these increased requirements of the over-all system more sophisticated, interfaces between the individual parts of this system are necessary.
A transceiver typically represents a fundamental part of a communication system. The digital transceiver comprises a base-band processor (base-band, BB-unit) and a radio transceiver unit (RF-unit). The base-band processor manages the base-band data stream and controls the function of the RF-unit. The RF-unit requires a special configuration and control apparatus for its function. In some implementations the RF-unit and the BB-unit of a digital transceiver are implemented on separate chips. This has both, advantages and disadvantages. In any case this also requires a sophisticated and well-defined interface between the RF-unit and the BB-unit in order to not reduce the required system performance.
Even if the interface between the RF-unit and the BB-unit—hereinafter referred to as RF-base-band-interface—is similar to single-chip solutions being entirely on the same chip it requires careful considerations.
For previous digital communication systems the performance requirements were much more relaxed and therefore a larger variety of different data and control interfaces emerged.
3. Related Art
In the prior art different kinds of data interfaces exist. Some of them are described below:
a) Unmodulated Coded Bit Stream:
Systems which employ relatively simple modulation schemes (i.e., schemes which do not require high computation power for modulation/demodulation) may have the modulator/demodulator implemented on the RF chip. An example of this is a Bluetooth system which might apply a relatively simple interface.
b) Analog Differential I/Q Interface:
Some systems (e.g. OFDM-systems) employ more complex modulation schemes which usually perform the modulation/demodulation digitally on the base-band chip. These systems consequently require an analog interface for the modulated signals. One way to perform this task is to split the signal in so called I-parts and Q-parts (whereas I-parts refer to in-phase parts and Q-parts refer to quadrature-parts) and to interface this signal as analog differential signal either at base band frequencies or at a (relatively low-) intermediate frequency (IF). This interface scheme is very common for relatively high data rate systems, e.g. a wireless LAN system.
c) Digital I/Q Interface:
These systems are similar to the systems having analog differential I/Q interfaces (see above under b). However these signals are interfaced digitally—either parallel or serial.
Besides the above mentioned interfaces for the received data and the transmitted data stream also an interface for the various control signals and clock signals for the communication system is required. In particular, these are the following signals: system clock signal, bus clock signal, data receive/transmit enable signal (RX/TX enable), power-down signal stand-by signal, reset signal, transmit power control signal, RSSI-signal (RSSI=received signal strength indicator), signal for the desired received/transmitted (RX/TX) frequency band, signal for the desired operation mode, signals for transceiver calibration (e.g. gain control, offset cancellation and equalization signals).
In the prior art there are also different kinds of control and clock interfaces. In traditional non-hopping communication systems the control signals can be transmitted relatively slowly. Even in traditional frequency-hopping systems the hopping rate is slow enough to allow the transfer of these control signals on a per-burst base. This scheme allows relatively high flexibility. Additionally this also requires only low computational capabilities on the RF transceiver side. On the other hand, however, the maximum hopping rate is limited by the speed of this control interface.
FIG. 1 shows a schematical timing diagram illustrating the data communication between the RF-unit and the base-band unit of a transceiver using an interface according to the state of the art and thus illustrating the above mentioned problem.
In FIG. 1 the control signal part is referenced with reference Number A and the data signal part is referenced with reference Number B, C. The data communication is carried out in a burst-like mode splitting the data stream in a plurality (here three) data bursts B, C. The control signals A contain all control information for performing the data communication and especially the frequency hopping, such as the hopping frequencies the beginning and/or the end of a data burst, the duration of a data burst, the distance between the end of one data burst and the beginning of the following data burst, etc. The duration of a control signal is reference with G.
The data bursts B represent the data communication within the transceiver that is the data communication (receiving and/or transmitting) between the RF-chip and the base-band chip of the transceiver. Reference number C is directed on the data communication of the transceiver via the wireless interface that is the bursts C represent the received and/or transmitted data via the wireless interface. These transmitted and received data C have different frequencies f1, f2, f3 bands from one burst to the other. This technique of burst-like data communications using different channels is well known in the art and is illustrated in FIG. 1 by means of the upper vertical axis referenced with the transmit and receive frequencies f_TX, f_RX, respectively. The different frequencies f1, f2 f3 define respective communications channels. This technique is also known in the relevant art as band hopping technique or frequency hopping technique.
As shown in FIG. 1 and mentioned above the data communication is being controlled by control signals A. These control signals A are typically provided by the base-band unit. Typically, there exist a slight overlap D between the timing of the control signals A and the corresponding data bursts B, C.
To perform the data communication within the transceiver a defined time gap E between one data burst B, C and the following data burst B, C is needed for the control signals A preceding the respective data burst B, C.
The prior art data communication concepts like the one described with respect to FIG. 1 all have the disadvantage that they do not allow at all the required short response times or at least only at the cost of a significant higher number of parallel interface lines. However, this is often not tolerable since this consequently goes along with significant higher costs of the transceiver and thus the whole communication system. Typically, this is also not accepted by the customers.
The above mentioned data communication concepts having a transceiver including at least two chips—a RF-unit and a base-band unit—as well as data, control and clock interfaces arranged in between these two chips are widely known and are described in the prior art for example in WO 98/15105, WO 02/05513 A1, U.S. Pat. No. 5,923,761, WO 02/056488 A3, WO 03/063461 A1, WO 99/18744, US 2004/0013177 A1.